forming holes in surface of wet concrete

I want to set a few decorative 4" slate tiles in a small concrete slab under an outdoor stair. The tiles will not be ready for weeks, and I'd like to pour the pad this weekend, so my plan was to cut some 4.25" squares from 1/4" plywood, and press these into the wet mix flush with the surface. After it cures, I'll glue some little handles on the back with 15 minute epoxy and pull them out, leaving cavities to glue/grout the tiles into later.

Is this a mistake? Will the wood tend to float out of the wet mix, or other basic problems? Any better ideas for how to do this?

I planned to cut the edges with a 10° bevel to let them release. Is that enough? How tightly will the concrete adhere to the wood, particularly the porous edge grain? I thought I might cork on a bit of paraffin. Would some other release agent be better? Will residue interfere with the later adhesion of the tiles?

Finally, how long should the concrete cure before pulling the plugs? This is in Seattle, so average temp will be 50°, roughly.